Plasma thyroxine levels and thyroid gland histology in Baltic salmon (Salmo salar L.) during smoltification

Abstract

Plasma thyroxine (T₄) levels in Salmo salar L. showed a clearly defined surge which coincided with a rapid rise in the ambient water temperature and with the period of natural seaward migration. It is sugggested that the elevation of T₄ levels has a major role in the completion rather than the onset of the smoltification in Baltic salmon parr. The body coloration (i.e., smolt or parr appearance) could not be correlated to the levels of plasma T₄ and silvering does not at all times reflect the degree of smoltification. A relationship between size (length and weight) and plasma levels of T₄ was not detected for smolting Baltic salmon parr. Increased circulating levels of T₄ is not reflected by an overall morphological change of the thyroid cells during smoltification.